1. Field of the Invention
The present invention relates generally to passive optical networks (PONs) and, more specifically, to providing a self-test mechanism in a PON.
2. Description of the Related Art
The passive optical network (PON) is increasingly gaining acceptance as an access network for delivering broadband services such as Internet access, digital television and telephone service, to residential and business subscribers (i.e., customers of the service provider). The essence of a PON is that nothing but optical fiber and passive components are found in the path between the central office and subscribers. A single fiber can run from the central office to a passive splitter located near a group of subscribers, such as a neighborhood or office complex, and individual fibers can run from the splitter to individual subscribers or sub-groups of subscribers. The International Telecommunications Union (ITU) and the Institute of Electrical and Electronics Engineers (IEEE) are two standards-making bodies currently developing PON standards. The ITU has adopted recommendations of the Full Service Access Networks (FSAN) organization, including G983.x, a specification sometimes referred to as “broadband PON” (BPON), and G984.x, a specification sometimes referred to as “gigabit PON” (GPON). The IEEE has also adopted Ethernet-based (i.e., IEEE 802.3-based) PON standards referred to as “Ethernet PON” (EPON) and “gigabit EPON” (GEPON). These standards and recommendations are well known to persons skilled in the art to which the invention relates and are therefore not described in further detail in this patent specification. Although the term GPON may be used herein for convenience with regard to embodiments of the present invention described below, the invention can be applied to any suitable PON technology.
As illustrated in FIG. 1, in a conventional arrangement for delivering services to subscribers, a PON 10 includes an Optical Line Terminal (OLT) 12 (also known as optical line terminator) optically coupled to a number of Optical Network Terminators (ONTs) 14 (also known as optical network terminals and optical network units) by an arrangement of optical fibers 16 that includes one or more optical splitters (not separately shown for purposes of clarity). Each ONT 14 can be connected to various customer premises equipment (CPE) 18, such as a residential local area network (LAN) router, telephone, television, etc.
In the downstream direction, i.e., data transmitted from OLT 12 (e.g., located at the central office) to an ONT 14 (e.g., located at a subscriber's premises), the data packets are broadcast from OLT 12 to all of ONTs 14, and an ONT 14 can select the data to receive by matching the address embedded in the data packets to a previously provisioned or learned address. In other words, an ONT 14 only “listens” to data packets having a matching address. Thus, OLT 12 can transmit data “downstream” to a particular or selected ONT 14 by addressing it to that ONT. In the “upstream” direction, i.e., data transmitted from an ONT 14 to OLT 12, the data packets are time-domain multiplexed. In GPON, the downstream address typically comprises both a conventional Ethernet Media Access Control (MAC) address as well as a GPON Encapsulation Method (GEM) Port-ID. The GEM Port-ID can be used as a Quality-of-Service (QoS) designator to address a priority queue on a particular ONT user-network interface (UNI) 20 (port), to which a CPE 18 is connected. Quality of Service (QoS) is, generally speaking, a term that refers to assigning data packets different priorities based upon the type of data they carry. For example, real-time streaming video (i.e., digital television) service and audio (e.g., telephone or music service) are generally assigned higher priorities than Internet Web browsing and e-mail. In this manner, demands placed upon the network by multiple subscribers simultaneously requesting different services are less likely to diminish any subscriber's individual perception of service quality. QoS differentiation is an important feature of GPON systems, as the primary QoS designator, the GEM Port-ID, is generally used to define the virtual or logical data communication paths between OLT 12 and the ONT UNIs 20. The dashed lines at the ends of fibers 16 in FIG. 1 conceptually illustrate this potential multiplicity of PON virtual paths.
The structure of an exemplary one of ONTs 14 is shown in FIG. 1, illustrating how each virtual path can be used for data communication via one of UNIs 20, to which CPE 18 such as a LAN router, telephone, television, etc., can be connected. The structure of only one ONT 14 is shown for purposes of clarity, but the others can have the same or a similar structure.
The PON 10 serves as an access network through which subscribers can access services provided by the service provider's aggregation network 22. Aggregation network 22 is typically an Ethernet-based network that employs virtual LANs (VLANs), conceptually indicated in FIG. 1 by the dashed lines at the ends of the Ethernet cable or other communication medium 24 connecting to OLT 12. One conventional function of OLT 12 is to cross-connect the VLANs with the PON virtual paths. A VLAN is an example of a principle known as tunneling. Tunneling refers to the transmission of data intended for restricted or private use within a network in such a way that the network routing nodes are unaware that the transmission is part of the restricted network.
The service provider commonly includes an administrative workstation 26 for performing operations, administration and management (OAM) functions, such as provisioning and troubleshooting ONTs 14 and CPE 18. Sometimes, a subscriber will contact the service provider to report a problem. The service provider's ability to perform provisioning and troubleshooting is limited by the service provider's access to the subscriber end of the network. In some cases, service personnel attempting to verify provisioning or troubleshoot a reported problem would be aided by connecting test equipment directly to the ONT 14 or CPE 18 at the subscriber premises. However, dispatching service personnel to the subscriber premises (which service providers sometimes refer to as a “truck roll”) is uneconomical and time-consuming. Accordingly, it is desirable to enable a service provider to perform as much testing as possible from a remote location, such as administrative workstation 26. The present invention addresses these problems and deficiencies and others in the manner described below.